I had the same problem last autumn.
My problem was the front wheel brake hose. The hose was clogged (it can do this due swelling, even when there are no foreign particles in the brake fluid system), an while the master cylinder had power to force the brake fluid through the hose into the piston, the brake was never really released properly. As a result, the brake pad would constantly touch the brake disc slightly, heating it up in higher speeds, causing vibration. As I slowed down or pulled over, the disc would cool down and the vibration disappear, so it took a while to realize the problem.
And it started occasionally at first as yours did, then appeared more often.
If it is a problem in a brake, it's easy to find out. Next time this happens, pull over and feel all the rims. One of them should be warmer than the others or even quite hot.
And/or you may want to wipe all the rims clean and drive around a few days. If any of the wheels has a slightly jammed brake, it's rim will be a lot dirtier from brake dust than the others.
If you suspect a certain brake, you might see a color changes on the edges of that brake disc from the heating.
Also does your brake pedal feel harder than usually? (This can be hard to notice as it gets harder gradually and you might get used to it. But if the brake jams badly, you'll surely notice it.)
If it is the brakes, you'll need to replce the hoses and pads. If you drive around a lot with a jammed brake, it can warp the brake disc from the heat, and you'll need to replace that too.
Hopefully this helps, at least to rule out one option.